[media] doc-rst: improve documentation for DTV_FREQUENCY

DTV_FREQUENCY

=============

Central frequency of the channel.

Frequency of the digital TV transponder/channel.

Notes:

.. note::

  #. For satellite delivery systems, the frequency is in kHz.

  #. For cable and terrestrial delivery systems, the frequency is in

     Hz.

1)For satellite delivery systems, it is measured in kHz. For the other

ones, it is measured in Hz.

  #. On most delivery systems, the frequency is the center frequency

     of the transponder/channel. The exception is for ISDB-T, where

     the main carrier has a 1/7 offset from the center.

2)For ISDB-T, the channels are usually transmitted with an offset of

143kHz. E.g. a valid frequency could be 474143 kHz. The stepping is

bound to the bandwidth of the channel which is 6MHz.

  #. For ISDB-T, the channels are usually transmitted with an offset of

     about 143kHz. E.g. a valid frequency could be 474,143 kHz. The

     stepping is  bound to the bandwidth of the channel which is

     typically 6MHz.

3)As in ISDB-Tsb the channel consists of only one or three segments the

frequency step is 429kHz, 3*429 respectively. As for ISDB-T the central

frequency of the channel is expected.

  #. In ISDB-Tsb, the channel consists of only one or three segments the

     frequency step is 429kHz, 3*429 respectively.

.. _DTV-MODULATION: